Submerged structures such as buried steel pipelines corrode as a result of the natural electrochemical activity at their surface. The electrochemical activity consists of an oxidation reaction, which is the corrosion process, and a reduction reaction of equal rate. The rates of these reactions are controlled by the electrochemical potential established at the submerged structure surrounding medium interface.
Protection systems to retard or prevent the corrosion reaction have been devised and are sometimes legally required. One type of protection system applies a periodic, full-wave or half-wave, rectified AC voltage between the submerged structure and a counter electrode, which is also submerged in the medium. This voltage creates a cathodic current polarizing the structure to more negative values and reducing the electron flow arising from the natural corrosion reaction distributed along the surface of the pipe or other submerged structure.
In order to determine the voltage which should be applied by the protection system between the submerged structure and the counter electrode, or in order to determine the level of protection given by an existing protection system, it is desirable to accurately measure the polarized potential which exists at the boundary region between the submerged structure and the surrounding medium. If the polarized potential generated by the protection system is insufficient, the buried structure will continue to corrode and if the polarized potential is excessive, then unnecessary and costly electrical energy will be consumed without providing proportionate protection to the submerged structure.
Conventionally the polarized potential is measured by measuring the average or DC voltage which exists between the submerged structure and a reference electrode inserted in the surrounding medium. However, such a measurement is subject to significant error because the current from the protection system causes a distributed IR drop in the surrounding medium. Additionally, other external sources also can cause errors by introducing stray currents. Therefore, the resultant voltage measured between the submerged structure and the reference electrode represents the algebraic sum of polarized potential components and the IR drop component from the protection system and all external sources.
It is therefore a principal object of the present invention to eliminate the IR drop component and the effects from external sources from the measured voltage in order to obtain a more accurate measurement of the polarized cell potential.
The conventional measuring techniques are also subject to inaccuracies due to interference from 60 Hz signals and harmonics thereof, usually from a power distribution system and from high frequency and random noise signals.
Accordingly it is another object of the invention to provide a measuring system which is insensitive to some of these interference sources and can signal the presence of others.